Investments (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investment [Line Items]
Average remaining maturities for short-term available for sale investments			7 months
Average remaining maturities for long-term available for sale investments			2 years
Proceeds from available-for-sale of investments	$ 116,400,000	$ 709,200,000	$ 678,500,000	$ 1,160,000,000
Gross unrealized losses on available-for-sale investments	2,932,000		2,932,000		$ 3,887,000
Gain on cost-method investments acquired by third parties	0		1,200,000
Other Assets [Member]
Investment [Line Items]
Cost method investments	19,100,000		19,100,000		$ 19,200,000
Other Income (Expense) [Member]
Investment [Line Items]
Realized gains on the sales of available-for-sale investments	0	1,000,000	700,000	1,600,000
Realized losses on the sales of available-for-sale investments	200,000	100,000	400,000	300,000
Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Investment [Line Items]
Cost-method investments, aggregate carrying amount, evaluated for impairment during the period			2,600,000
Cost method investments, fair value after impairment			1,200,000
Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Other Income (Expense) [Member]
Investment [Line Items]
Cost-method Investments, Other than Temporary Impairment	$ 0	$ 900,000	$ 1,400,000	$ 1,100,000